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Is the compression ratio in a stock C3 engine bad for gas? (MPG wise)
if so, if you change the heads for a higher compression will you save gas? ( it it noticeable or just 1 MPG is added)
 

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higher compression does increase mpg, but, too high and you need premium fuel..changing heads to gain mpg , not really effective..you would be better off going to an overdrive trans, you can get 20-30% mpg gains hiway and extend the life of the motor
 

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Is the compression ratio in a stock C3 engine bad for gas? (MPG wise)
if so, if you change the heads for a higher compression will you save gas? ( it it noticeable or just 1 MPG is added)

A higher compression ratio will yield more horsepower and it'll also increase your gas mileage slightly. But as a higher compression ratio requires a higher octane of gasoline that is 5% more expensive you end up with a wash. But yes, a higher compression ratio will increase your gas mileage. Maybe 1-2 mpg.
 

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... I already have my eyes set on a conversion kit from monster transmissions for the mpg's

A TH700R4 is the only way to go. I just put one in my big block '71 and it's positively delightful. My tachometer only reads 1600 rpm at 60 mph. And Fishslayer also has one in his '81 383" stroker.
 

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You're not doing this to actually save money, are you?
Because if you spend, say $1500 total on heads and pick up a full 2mpg while still being able to run on regular gas, you will need to drive your car for 30,000 miles before you can even break even, and that is making some pretty generous assumptions for the cost of the heads, the cost of gas, and what fuel economy you have and can get with them! Modding for fuel economy never pays off... Mod for power: it's easier to justify.
Drop another 1500 on gears and now its 60,000 trouble-free miles before break even...
How many miles do you drive in a year? ;)
 

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does higher compression really hurt mileage or is it the heavy foot that evolves from it ?
 

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The big three spend millions of dollars every year trying to increase fuel economy just 1%.
MANY millions. About 5 of those millions get spent right where I work at an internal combustion research laboratory: there's an enormous difference here though: they are not doing it to save gas: car companies don't worry about what gasoline costs, they care about how many cars they can sell. They improve fuel economy because it boosts sales and that makes money. You spending $1500 to save 50 bucks on every 1000 miles driven does not save money. GM spending 1.5mil to make a truck get 0.5mpg better than the competition can earn them the number 1 spot on sales and earn many millions in profits.
 

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MANY millions. About 5 of those millions get spent right where I work at an internal combustion research laboratory: there's an enormous difference here though: they are not doing it to save gas: car companies don't worry about what gasoline costs, they care about how many cars they can sell. They improve fuel economy because it boosts sales and that makes money. You spending $1500 to save 50 bucks on every 1000 miles driven does not save money. GM spending 1.5mil to make a truck get 0.5mpg better than the competition can earn them the number 1 spot on sales and earn many millions in profits.

I recently put a TH700R4 in my big block '71 for the sole purpose of increasing my fuel mileage. Altogether I have spent about $1700 for the conversion and I calculated it'll take me about 18,000 to 20,000 miles of driving to break even. So it costs money to save money.
 

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I recently put a TH700R4 in my big block '71 for the sole purpose of increasing my fuel mileage. Altogether I have spent about $1700 for the conversion and I calculated it'll take me about 18,000 to 20,000 miles of driving to break even. So it costs money to save money.
What was your before/after mpg?
 

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What was your before/after mpg?
its not just the increase in mpg, which by the way went from 15-16 with a TH350, to 19-20 with a 700r4,and thats on my stroker. You would get near 24-25 with stock motor..but , its also the 30 % less rpm at hiway speeds. That reduces wear and tear and extends overall engine life. if you get 200,000 miles out of an engine instead of 160,000, HOW MUCH DID THAT SAVE YOU?
 

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its not just the increase in mpg, which by the way went from 15-16 with a TH350, to 19-20 with a 700r4,and thats on my stroker. You would get near 24-25 with stock motor..but , its also the 30 % less rpm at hiway speeds. That reduces wear and tear and extends overall engine life. if you get 200,000 miles out of an engine instead of 160,000, HOW MUCH DID THAT SAVE YOU?
I can't imagine driving anything for that long. But if he really gained 4mpg for spending 1700 I can see breaking even in 20000 miles or so. Of course that's kind of odd because all the C5/C6 guys go from the factory 3.42 gears to 3.90 or 4.10 final drives and they all claim to lose no more than "1 or 2mpg"
How many miles do you drive that car for in one year toobroke?
 

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going from a 3.42 to a 3.90 is not a 30% increase .jump that 3.42 to a 4.56, and see how much mpg s you lose! whereas going from a 1:1 final drive trans to a 30% reduction overdrive trans and a lock-up converter [ which his C3 did not have from factory] will gain that much increase in mpg or more...add to that, the rapidly increasing cost of a gallon of gas, it will pay for itself fairly soon. Not to mention the added benefit of the performance increase if you desire to use it..the 700r4 has a 3.06 first gear ratio, you will feel that immediately, should you chose to. Converting a 3spd a/t to AOD is the most cost effective way to reduce fuel cost.
 

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I suspect you are correct sir....lol
 
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